High slip rate for a low seismicity along the Palu-Koro active fault in central Sulawesi (Indonesia)

High slip rate for a low seismicity along the Palu-Koro activefault in central Sulawesi (Indonesia)

Olivier Bellier,1 Michel SeÂbrier,2* Thierry Beaudouin,2 Michel Villeneuve,3 ReÂgis Braucher,4

Didier BourleÁ s,4 Lionel Siame,2 Eka Putranto5 and Indyo Pratomo5

1CEREGE (Centre EuropeÂen de Recherche et d'Enseignement des GeÂosciences de l'Environnement), UMR CNRS 6635 ± UniversiteÂ Aix-

Marseille III, BP 80, EuropoÃle MeÂditerraneÂen de l'Arbois, 13545 Aix en Provence Cedex 4; 2UMR CNRS 8616 `ORSAYTERRE, UniversiteÂ

de Paris-Sud, 91405 Orsay Cedex; 3UMR-CNRS 6019, University de Provence, 13331 Marseille; 4CEREGE, University Aix-Maseille III,

France; 5DGGMR/PPPG (Geological Research and Development Centre), Bandung, Indonesia

Introduction

Sulawesi Island, eastern Indonesia, isat the triple junction of the Paci®c(through the Philippine plate), Indo-Australian and Eurasian Plates(Fig. 1). Its peculiar shape results froman on-going, complex history of colli-sions and subsequent rotations ofcontinental slivers, island arcs andoceanic domains with the SundaBlock, i.e. the south-eastern edge ofthe Eurasian Plate. Presently, Sulawesiis obliquely crossed by one of the mainfault systems limiting the EurasianPlate to the East: the left-lateral Cen-tral Sulawesi Fault System (CSFS).This fault system, which comprisestwo fault zones, the NNW-trendingPalu-Koro (PKF) and the WNW-trending Matano (MF) fault zones,connects, from north-west to south-east, the North Sulawesi Subductionzone (NSS) to the Banda Sea domain(Fig. 1). Previous studies have sugges-

ABSTRACT

In eastern Indonesia, the Central Sulawesi fault system consistsof complex left-lateral strike-slip fault zones located within thetriple junction area between the Paci®c, Indo-Australian andEurasian plates. Seismicity in Central Sulawesi documents low-magnitude shallow earthquakes related, from NW to SE, to theNNW-trending Palu-Koro (PKF) and WNW-trending Matano faultzones. Study of the active fault traces indicates a northwardgrowing complexity in the PKF segmentation. Left-lateraldisplacement of 370 � 10 m of streams incised within fans,

whose deposition has been dated at 11 000 � 2300 years,yields a calculated PKF horizontal slip rate of 35 � 8 mm yr)1.This geologically determined long-term slip rate agrees with thefar-®eld strike-slip rate of 32±45 mm yr)1 previously proposedfrom GPS measurements and con®rms that the PKF is a fastslipping fault with a relatively low level of seismicity.

Terra Nova, 13, 463±470, 2001

Fig. 1 Seismicity of the Sulawesian region from the ISC and USGS catalogues. PKFand MF represent the Matano Fault and Palu Koro Fault zones, respectively, whileNSS is the North Sulawesi Subduction. The box indicates the approximate location ofFig. 2. Inset at bottom right shows the SE Asia geodynamic frame and velocity ®eld(arrows) of the Indo-Australian and Philippine plates relative to Eurasia (afterBaroux et al., 1998).

Correspondence: Olivier Bellier, CEREGE

(Centre EuropeÂ en de Recherche et d'Ens-

eignement des GeÂ osciences de l'Environn-

ement), UMR CNRS 6635 ± UniversiteÂ

Aix-Marseille III, BP 80, EuropoÃ le MeÂ di-

terraneÂ en de l'Arbois, 13545 Aix en Pro-

vence Cedex 4, France. Tel.: +33 04 42 97

1660; fax: +33 04 42 97 1559, e-mail:

[email protected]

*Now at: ESA CNRS 7072, `Tectonique',

University Pierre et Marie Curie, 75252

PARIS cedex O5, France.

Ó 2001 Blackwell Science Ltd 463

ted major active tectonics along theCSFS, particularly along the PKF(Tjia, 1969; Katili, 1970). However,the seismicity of Sulawesi, as recordedby global seismic networks (Fig. 1),Indonesian network and historicalseismicity indicate a relatively low levelof shallow seismic activity in CentralSulawesi (e.g. Beaudouin, 1998). Incontrast with this low seismicity, PKFlong-term slip rates, as high as 40±50 mm yr)1 for the last 5 Myr, havebeen estimated, on the basis of geody-namic reconstruction models (i.e. plateconvergence rate along the NSS; Silveret al., 1983; Walpersdorf et al., 1998a)or from palaeomagnetic studies (Sur-mont et al., 1994). Based on a 5-Myrtime span, these estimates may integ-rate different tectonic regimes andconsequently variations of the slip rateon the PKF. In that case, the estimatesare most likely inaccurate, particularlyin a rapidly evolving complex area of atriple junction. Recently, the far-®eldslip rate of the PKF was calculatedat � 38 � 8 mm yr)1 from geodetic(GPS) measurements (Walpersdorfet al., 1998b; Stevens et al., 1999). Tocomplement these analyses, it is there-fore crucial to estimate the PKF long-term slip rates to interpret GPS results.The aim of the current study was todetermine the Holocene slip rate,i.e. on a time-span during which thetectonic regime has been stable.To constrain the PKF long-term

slip rate, detailed mapping of theactive fault traces and of the along-strike o�sets have been performedusing SPOT images. They were furtherconstrained by geomorphic and tec-tonic ®eld studies and complementedby analyses on topographic and geo-logical maps (GRDC, 1981±93). Thesepermitted us to determine a horizontallong-term slip rate on the PKF,over the Holocene, by observationsof stream o�sets. The o�sets were

Fig. 2 Active segmentation of the PKF.The boxes indicate the approximatelocations of Figs 3, 5 and 7. LeboniRBF, the Leboni fault relay-bend zonethat connects the PKF to the MF.Dashed faults are those with a dominantnormal component (ticked on the down-thrown block) while triangles indicatefaults with a reverse component (trian-gles on the upthrown block).

High slip rate along a low-seismicity fault · O. Bellier et al. Terra Nova, Vol 13, No. 6, 463±470.............................................................................................................................................................

464 Ó 2001 Blackwell Science Ltd

correlated to Recent alluvial fan aban-donment dated using in situ produced10Be concentrations.

Historical and instrumentalseismicity in Central Sulawesi

According to the seismicity catalogues(USGS, ISC, CMT, etc.), Central Su-lawesi seismicity is di�use and charac-terized by few shallow events locatedaround both the PKF and the MFzones (Fig. 1). Although poorly docu-mented, historical seismicity in CentralSulawesi has been recorded since the19th century. However, even if somedamaging earthquakes have beenreported within Central Sulawesi, few

major earthquakes (Mw > 4.5) haveoccurred on the PKF and MF zonesover at least the 100 years. Katili(1970) mentioned three earthquakesnear the trace of the PKF in theGimpu (1905), Kulawi (1907) andKantewoe (1934) basins, respectively.A damaging earthquake on the PKF in1909 was reported by Hamilton(1979). In addition, a recent reapprais-al study (Beaudouin, 1998) of theseismic history based on several cata-logues (RotheÂ , 1969; Kertapati et al.,1991; PPPG, 1995; Pelinovsky et al.,1996) reported 28 major earthquakesaround Sulawesi between 1845 and1998. This study did not report eventsin 1905, 1907, 1909 and 1934 but

reported two destructive earthquakesin 1927 and 1985 that could haveoccurred along the PKF and two othermajor events in 1968 (Ms � 6.7) and1993 (Mw � 5.7) that probably cor-respond to PKF reactivations.

Fault geometry in Central Sulawesi

A large-scale map covering the CSFSwas compiled by investigation of de-tailed geomorphological fault charac-teristics. It provides evidence for abroad geometric discontinuity ar-ranged in a complex crustal-scale faultrelay-bend zone located at around2°15¢S (Leboni RBF, Fig. 2). This zoneconnects the PKF to the MF (Fig. 1)that accounts for the major activedeformation in Central Sulawesi.

The Palu-Koro Fault segmentation

To examine potential partitioning ofthe deformation across the Palu-Kororegion, detailed mapping of the sur-face fault traces was conducted. Thisprovides evidence for discontinuitiessuch as stepovers, relays and bendsand allowed us to compile a segmen-

Fig. 3 Detailed view of the SPOT image K/J-309/353 (22 October 1988). Location onFig. 2. Arrows point to the PKF fault trace. S1, S2, S3, S4 are the PKF fault segmentsreported on Fig. 2. The box approximately locates the Moa river (Fig. 5).

Fig. 4 Present-day fault kinematics ofthe northern PKF. Examples of strike-slip faulting slip data measured alongthe major PKF bounding the Palu basin:lower hemisphere stereoplot of faultplanes and measured striations, direc-tion arrows point in the horizontal slipazimuth direction. r1, r2 and r3 repre-sent the stress axes of the regionallysigni®cant stress regime obtained fromthe inversion of major fault striationsmeasured around the Palu bassin(Beaudouin, 1998). Large open andblack arrows give the azimuths of thecomputed r3 and r1, respectively.

Terra Nova, Vol 13, No. 6, 463±470 O. Bellier et al. · High slip rate along a low-seismicity fault.............................................................................................................................................................


tation map (Fig. 2). The map docu-ments seven geometric fault segmentswhose lengths range from 15 to59 km (segment S0 to S6). SegmentS0 is related to the northern segmentcomprised within the Leboni relay-bend marking the PKF southerntermination. S6 is the northernmostonshore PKF segment, its northerntermination being o�shore. The PKFsegment lengths are: S0: 15 km, S1:59 km, S2: 43 km, S3: 29 km S4:40 km, S5: 20 km, S6: � 12 km (on-shore). Based on this analysis, distinctdomains may be distinguished. The

northern domain is characterized bysegments smaller than the southernone. Indeed, the southern PKF de-formation is localized along the linearand long segments, S1 and S2(Fig. 3). The northernmost PKF do-main comprises several segments ar-ranged in an en echelon pattern or inrelay. In the northernmost zone thedeformation is distributed over abroad zone and extends into the PaluGulf. Thus, this large-scale segmenta-tion study provides evidence for anorthward-growing complexity in thesegmentation.

Palu-Koro Fault kinematics

The PKF exhibits left-lateral o�sets ofgeomorphic features, i.e. streams, lateQuaternary alluvial fans, etc. Facetedspurs with 300±400-m-high triangularfacets also characterize the northernPKF trace. The western Palu basinboundary front is controlled by a N-to NNW-trending, 2300±2500-m-highescarpment. This escarpment as wellas Quaternary coral reef terraces thathave been uplifted by 210 m indicatethat a dip-slip component character-izes the PKF present-day faulting.These observations show that bothstrike-slip and normal-slip faultingpresently occur along the northernPKF. This implies a transtensionaltectonic regime con®rmed by analysisof the slip-vector measured on themajor fault planes related to recentfaulting (Fig. 4) and by GPS meas-urements (Walpersdorf et al., 1998b).However, both methods yield a dom-inant lateral component for the PKFpresent-day faulting (rake of � 15°)that agrees with a NW-trending r1

strike-slip stress regime (Fig. 4). Faultkinematics as well as ®ssion trackanalyses provide evidence for a changein the tectonic regime since 6 Ma(Beaudouin, 1998; Bellier et al.,1998). They demonstrate that the2500-m-high northern PKF escarp-ment is partly inherited from an olderextensional (normal slip faulting) tec-tonic regime.

Geomorphic feature offsets alongthe Palu-Koro Fault

Along the PKF trace, morphologicalfeatures are left-laterally o�set andmight thus be used to calculate thefault strike-slip rate. For along-strikePKF segments S1 and S2, left-lateralstream o�sets ranging from210 � 20 m to 3530 � 20 m havebeen observed on SPOT images (usu-ally, the stream o�set uncertaintiesranged between � 10 m and � 20 m,e.g. Bellier and SeÂ brier, 1995). Thelonger o�set of 3530 � 20 m is rela-ted to the major Moa river (at lat.� 1°45¢S; Fig. 5) that incised Pleisto-cene magmatic rocks, 1.62 Ma in age(Sukamto, 1975). However, it is di�-cult to estimate the age of this riveremplacement except to say that itis related to major recent climaticevents.

Fig. 5 (a) Detailed views of SPOT image K/J-309/353 showing the Moa river streamo�set. (b) Interpretation sketch of the SPOT view reported on (a). PKF (S1)represents the fault trace of PKF segment S1.



From analysis of SPOT imagescovering the PKF zone, we selectedto study the Palu basin region wherethe morphological characteristics ofthe fault indicate high current tectonicactivity. It was selected for three mainreasons: (1) several geomorphic fea-tures o�sets are observed; (2) contraryto most locations along the PKF, ®eldaccessibility to this zone is less limitedby a dense forest; (3) it also o�ers theopportunity, through analysis of thetermination of the PKF major seg-ment, to estimate the seismic risk closeto Palu city, the largest city in CentralSulawesi. Fan edges and streams inci-sing Quaternary deposits along thewestern border of the Palu basinexhibit left-lateral o�sets ranging from65 � 20 m to 420 � 20 m (Fig. 6).The average long-term slip rate alongthe PKF has thus been determinedthrough the dating of Late Quater-nary alluvial fan abandonment. In-deed, the observed cut and ®ll-inprocess of emplacement strongly sug-

gests that the o�set channel networksof the di�erent fan units have beenformed just after their abandonment.In that case, alluvial fans, which areindicators of climatic change, may beused not only as markers allowingidenti®cation of cumulative tectonicdisplacements through their o�setstreams, but also to estimate directlythe PKF horizontal displacement sliprate through the dating of the alluvialfan surface abandonment using in situproduced 10Be (e.g. Siame et al., 1997,2001).

Quaternary alluvial fans locatedon the north-western borderof the Palu basin

Late Quaternary alluvial fans ana-lysed in this study are located in thewestern front of the Palu basin, atlatitudes ranging between � 0°50¢ and� 1°10¢S (Fig. 7). They have beenstudied using combined cartography,geomorphic (SPOT and ®eld) analy-

ses, and 10Be exposure age approa-ches. Observation of the relativespatial relationships between the allu-vial fans, and thus their relative chro-nology, leads us to subdivide thealluvial fans into two units. Indeed,east of the fault scarp there are twogenerations of incised alluvial fans;the younger, A1, is cone shaped inplan view. During deposition, the A1fans truncated a part of older fans,A2, which are presently mainly ob-served as relics (Fig. 7) (Bellier et al.,1999a).

Ages of the fan systems

The minimum exposure ages havealready been extensively discussed inBellier et al. (1999a). Measurements ofin situ produced 10Be concentrations inquartz boulders exposed on top fansurfaces show that they have beenemplaced during the last two succes-sive major climatic events, at theinitiation of deglaciation ending the

Fig. 6 (a) Detailed view of panchromatic SPOT image K/J-309/352 showing examples of stream and fan boundary o�sets.(b) Interpretation sketch of the SPOT view reported on (a). Arrows point to the horizontal o�sets. Location on Fig. 7.



last two glacial cycles. For the strati-graphically determined younger allu-vial deposit (A1), the measured 10Beconcentration yields a minimum expo-

sure age of 11 000 � 2300 years. Theinterpretation of this age is that theyounger fan surface was stabilized atthe initiation of the deglaciation end-ing the Last Glacial Maximum (i.e.end of stage 2). The minimum expo-sure age calculated for the older fansurface A2 leads us to propose that theevent that induced this surface em-placement may have been deglaciationending isotopic stage 6, before com-mencement of the last interglacialconditions (� 125 � 20 ka).

Long-term slip rate alongthe Palu-Koro Fault

Displacements of fan edges by thePKF have been observed. However,fan boundaries are often di�cult tomap precisely and likely observationmis®ts induce large uncertainties.Along the northern PKF, the observedfan o�set ranges from 280 � 20 m to420 � 20 m (A, Fig. 6). The youngerfan age of 11 000 � 2300 years yieldsa slip rate ranging between 20 and51 mm yr)1. Since the last major re-gional climate change is contempora-neous with the deglaciation of the LastGlacial Maximum, the stream incisionmost likely results from enhancedstream power associated with intergla-cial conditions. In addition, owing tothe high erosion rate under a tropicalclimate, the assumption that the minorstream network within the Palu basinboundary was installed just after thefan emplacement implies that the fanenvelope was incised by distinctstream channel networks 11 000 �2300 years ago. These stream net-works reveal left-lateral offsets ran-ging between 65 � 20 m and370 � 20 m along the PKF (Figs 6and 8). The maximum stream offset of370 � 20 m (B, Fig. 6) is related to adrainage in capture process (see below)and re¯ects in fact an A1 fan edgeoffset (presently eroded and preservedas a relic outcrop, see Fig. 6). Thisoffset allows us to calculate a hori-zontal displacement rate of 36 �9 mm yr)1. North of this, a minordrainage network provides evidencefor systematic stream offsets rangingfrom 65 m to 160 m. Differentamounts of incision characterize theanalysed upstream and downstreamchannels, and thus re¯ect stream cap-ture (e.g. Bellier et al., 1999b). Takingthis into account, the `true' cumulative

river offset deduced from the drainagenetwork restoration (Fig. 8) is 370 �10 m. This yields a slip rate of35 � 8 mm yr)1 that better con-strains the slip rate deduced from theimprecise fan and stream offsets.In addition, the assumption that the

mountain belt major stream networka�ected by the PKF segments S1 andS2 could have been installed coevalwith the A2 older fan emplacement, at� 125 � 20 ka, allows us to calculatea slip-rate using the Moa river streamo�set of 3530 � 20 m. This o�setyields a slip rate of � 29 �5 mm yr)1. Even if speculative, it isconsistent with the slip-rate deter-mined using offsets affecting Recentfeatures.

Conclusion

The aim of this geological survey wasto identify and quantify the activedeformation of the northern part ofthe PKF over the period of the Holo-cene. The determined long-term slip of� 35 � 8 mm yr)1 is in fairly closeagreement with the GPS determinedslip rate (Walpersdorf et al., 1998b;Stevens et al., 1999). It con®rms thatthe PKF is a fast slipping fault with arelatively low level of seismicity. Thissuggests that either: (1) the fault ismechanically locked or (2) part of thefault displacement is aseismic. Hypo-thesis 1 would imply that the faultdisplacement is purely coseismic,produced by major earthquakes.Preliminary palaeoseismicity results(Beaudouin, 1998; Bellier et al., 1998)do not support such an interpretationbecause they provide evidence forthree 6.8 < Mw < 8 earthquakesalong the PKF segment during thelast 2000 years, implying a recurrenceinterval of about 700 years for Mw� 7±8 earthquakes. Even if the upperbound magnitude is chosen with ahorizontal displacement per event ofabout 10 m (e.g. Wells and Copper-smith, 1994), those three events wouldhave produced a cumulative offsetof � 30 m whereas the � 35 �8 mm yr)1 long-term lateral slip rateyields an offset ranging between 54and 86 m within 2000 years. Hypo-thesis 2 would imply that the prelim-inary palaeoseismicity results areconsistent with a coseismic slip de®cit.In such a case, a signi®cant part of thefault displacement would be accom-

Fig. 7 Map of the Quaternary alluvialfan deposits from ®eld observations andPanchromatic SPOT image (KJ 309-352)interpretations (simpli®ed after Bellieret al., 1999a). It shows the geometricalrelationships of the alluvial fan unitsalong the PKF (see Fig. 2), from younger(A1, dated at � 11 ka) to older (A2,dated at � 125 ka). The fault tracerepresents the northern PKF. The boxshows the fan (A) and stream (B) offsetspresented on Fig. 6. SA.2 and SA.1are the sampling sites for A2 and A1,respectively (see Bellier et al., 1999a).Location: see Fig. 2.



modated by creep, which is not pres-ently evidenced by available GPS data(Walpersdorf et al., 1998b; Stevenset al., 1999).

Acknowledgments

This study was developed in the frameworkof an ASEAN-EU project, the `GEODYS-SEA project'; a portion of the programmewas set apart for studying active faulting inSulawesi. 10Be analyses were performedby accelerator mass spectrometry at theTandeÂ tron AMS facility, Gif-sur-Yvette,France. SPOT images were providedthanks to the ISIS programme support(CNES).

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Received 11 September 2001; revised versionaccepted 6 November 2001



Documents

High slip rate for a low seismicity along the Palu-Koro active fault in central Sulawesi (Indonesia)